Despite the certainty many seem to feel about the causes, effects
and extent of climate change, we are in fact making only slow
progress in our understanding of the underlying science. My old
professor at Harvard, the great economist Joseph Schumpeter, used to
insist that a principal tool of economic science was history -- which
served to temper the enthusiasms of the here and now. This must be
even more so in climatological science. In recent years the
inclination has been to attribute the warming we have lately
experienced to a single dominant cause -- the increase in greenhouse
gases. Yet climate has always been changing -- and sometimes the
swings have been rapid.

At the time the U.S. Department of Energy was created in 1977,
there was widespread concern about the cooling trend that had been
observed for the previous quarter-century. After 1940 the
temperature, at least in the Northern Hemisphere, had dropped about
one-half degree Fahrenheit -- and more in the higher latitudes. In
1974 the National Science Board, the governing body of the National
Science Foundation, stated: "During the last 20 to 30 years, world
temperature has fallen, irregularly at first but more sharply over
the last decade." Two years earlier, the board had observed: "Judging
from the record of the past interglacial ages, the present time of
high temperatures should be drawing to an end . . . leading into the
next glacial age." And in 1975 the National Academy of Sciences
stated: "The climates of the earth have always been changing, and
they will doubtless continue to do so in the future. How large these
future changes will be, and where and how rapidly they will occur, we
do not know."

These statements -- just a quarter-century old -- should provide
us with a dose of humility as we look into the more distant future. A
touch of that humility might help temper the current raging
controversies over global warming. What has concerned me in recent
years is that belief in the greenhouse effect, persuasive as it is,
has been transmuted into the dominant forcing mechanism affecting
climate change -- more or less to the exclusion of other forcing
mechanisms. The CO2/climate-change relationship has hardened into
orthodoxy -- always a worrisome sign -- an orthodoxy that searches
out heretics and seeks to punish them.

We are in command of certain essential facts. First, since the
start of the 20th century, the mean temperature at the earth's
surface has risen about 1 degree Fahrenheit. Second, the level of CO2
in the atmosphere has been increasing for more than 150 years. Third,
CO2 is a greenhouse gas -- and increases in it, other things being
equal, are likely to lead to further warming. Beyond these few facts,
science remains unable either to attribute past climate changes to
changes in CO2 or to forecast with any degree of precision how
climate will change in the future.

Of the rise in temperature during the 20th century, the bulk
occurred from 1900 to 1940. It was followed by the aforementioned
cooling trend from 1940 to around 1975. Yet the concentration of
greenhouse gases was measurably higher in that later period than in
the former. That drop in temperature came after what was described in
the National Geographic as "six decades of abnormal warmth."

In recent years much attention has been paid in the press to
longer growing seasons and shrinking glaciers. Yet in the earlier
period up to 1975, the annual growing season in England had shrunk by
some nine or 10 days, summer frosts in the upper Midwest occasionally
damaged crops, the glaciers in Switzerland had begun to advance
again, and sea ice had returned to Iceland's coasts after more than
40 years of its near absence.

When we look back over the past millennium, the questions that
arise are even more perplexing. The so-called Climatic Optimum of the
early Middle Ages, when the earth temperatures were 1 to 2 degrees
warmer than today and the Vikings established their flourishing
colonies in Greenland, was succeeded by the Little Ice Age, lasting
down to the early 19th century. Neither can be explained by
concentrations of greenhouse gases. Moreover, through much of the
earth's history, increases in CO2 have followed global warming,
rather than the other way around.

We cannot tell how much of the recent warming trend can be
attributed to the greenhouse effect and how much to other factors. In
climate change, we have only a limited grasp of the overall forces at
work. Uncertainties have continued to abound -- and must be reduced.
Any approach to policy formation under conditions of such uncertainty
should be taken only on an exploratory and sequential basis. A
premature commitment to a fixed policy can only proceed with fear and
trembling.

In the Third Assessment by the International Panel on Climate
Change, recent climate change is attributed primarily to human
causes, with the usual caveats regarding uncertainties. The record of
the past 150 years is scanned, and three forcing mechanisms are
highlighted: anthropogenic (human-caused) greenhouse gases, volcanoes
and the 11-year sunspot cycle. Other phenomena are represented
poorly, if at all, and generally are ignored in these models. Because
only the past 150 years are captured, the vast swings of the previous
thousand years are not analyzed. The upshot is that any natural
variations, other than volcanic eruptions, are overshadowed by
anthropogenic greenhouse gases.

Most significant: The possibility of long-term cycles in solar
activity is neglected because there is a scarcity of direct
measurement. Nonetheless, solar irradiance and its variation seem
highly likely to be a principal cause of long-term climatic change.
Their role in longer-term weather cycles needs to be better
understood.There is an idea among the public that "the science is
settled." Aside from the limited facts I cited earlier, that remains
far from the truth. Today we have far better instruments, better
measurements and better time series than we have ever had. Still, we
are in danger of prematurely embracing certitudes and losing
open-mindedness. We need to be more modest.

(*) The writer, who has served as secretary
of energy, made these comments at a symposium on the 25th anniversary
of the Energy Department's C02/climate change program.